From the amino acid composition of gluten proteins and the substrate specificity of transglutaminases (TGase) we concluded that gluten proteins can be favourable substrates for TGases due to their high glutamine content. By use of sodium dodecyl sulfate polyacrylamide gel-electrophoresis it was demonstrated that from gluten-ES and gluten-TS high-molecular-weight proteins developed in the presence of Ca2+ and red blood cell lysate containing TGase. When ovalbumin or deamidated gluten were applied as substrates no high-molecular-weight products were formed. Upon spectrophotometric measurements we found that covalent cross-links (isopeptide bonds) formed under the effect of TGases presumably cause a change in the position of chromophore groups in the substrates. Absorption decrease was detected between 274-276 nm as a result in the case of gluten-TS and gluten-ES used as substrates for TGase. No such change occurred in ovalbumin and deamidated gluten, applied as controls, under the influence of TGase. On the basis of our experiments it is postulated that the first step in gluten toxicity is presumably the binding of gluten to the intestine mucosa. In this binding the high transglutaminase activity in the intestines of coeliac patients and the high glutamine content of gluten may have an important role.